Mechanical devices and, more particularly, aircraft contain various types of fluid systems, as for example, hydraulic systems. These hydraulic systems perform various functions including the control of flight control surfaces of the aircraft. It is critical for these hydraulic systems to contain an adequate supply of hydraulic fluid in their reservoirs so that they may function properly. Otherwise, if there are insufficient fluid levels, such hydraulic fluid systems may partially or completely fail to operate. The failure of such hydraulic fluid systems may render the aircraft uncontrollable with potentially disastrous results. To insure adequate hydraulic fluid levels, on board instrumentation is typically provided to monitor the fluid levels both during flight, by aircraft operating personnel, and on the ground during periodic maintenance by maintenance personnel.
In an aircraft hydraulic fluid system, the reservoir containing the hydraulic fluid may be located in an area that is only accessible by the disassembly of aircraft components in the vicinity thereof. Such disassembly is complex, time consuming, expensive and inefficient for merely determining the fluid level in the reservoir. Therefore, in order to overcome this problem, a remote fluid level indicator is provided comprising a first portion for detecting the fluid level and an indicator portion, which is conveniently located, for displaying the fluid level. In general, a remote fluid level indicator is provided for each hydraulic fluid system, with the indicating portion of each indicator located in the cockpit of the aircraft so that aircraft personnel may remotely monitor the fluid level. While this type of fluid level indicator may be conveniently located for aircraft personnel, such indicators are, however, inconveniently located for maintenance personnel who typically service the aircraft from outside the cockpit, such as underneath the aircraft. Moreover, some fluid level indicators comprise an electrical transducer and rely on an aircraft energy source to measure the fluid level of the reservoir. In such a fluid level indicator, the measured fluid level is converted to an electrical signal for display on a cockpit gauge. This type of fluid level indicator is undesirable for maintenance personnel since during one-the-ground maintenance, the electrical power source is generally deenergized rendering such indicators inoperative.
Furthermore, it will be appreciated that the volume level of the hydraulic fluid varies in accordance with the temperature thereof, i.e., as the fluid temperature increases the volume of the fluid increases and vice versa. Accordingly, to determine the amount of fluid in the reservoir with a higher precision, the fluid indicator should compensate for actual fluid temperature relative to a predetermined reference temperature.
Heretofore, various attempts have been made to provide a remote liquid level indicator. For example, U.S. Pat. No. 3,129,125 to Vasel is directed to a liquid level indicator for measuring the volume of the fluid contained in the reservoir and comprises a plurality of transilluminators having first ends illuminated by a light source and second ends terminating at an indicating panel for indicating the liquid level. Each of the plurality of transilluminators passes into and out of the reservoir. The plural transilluminators are each separated into first and second portions with a respective prism therebetween. Each prism is located at a corresponding elevation in the reservoir, so that as the fluid level increases the number of prisms immersed thereby also increases. In this arrangement, if a prism is not covered by the fluid, the light transmitted from a corresponding first portion of the transilluminator is refracted to the second portion thereof for transmission to the indicating panel, thus illuminating the other end of the transilluminator. Alternatively, if a prism is covered by the fluid, the angle of refraction is changed such that the light is not refracted to the second portion, thus not illuminating the other end of the transilluminator. In this device, as the fluid level increases the number of illuminated receiving ends decreases, and as the fluid level decreases the number of illuminated receiving ends increases. However, this device does not address the problem of variations in the fluid volume due to changes in fluid temperature. Moreover, since the transilluminators pass into and out of the reservoir with the prisms internally located, the reservoir is complicated to construct and maintain.
U.S. Pat. No. 3,119,268 to Stanley is directed to a liquid level indicating gauge for use in a high pressure steam boiler comprising a liquid-containing column having a water column chamber spaced within a confinement wall. The water column has two opposite sides, each side comprising a series of vertically aligned glazed openings for transmitting light therethrough. The column also comprises a series of shutters for each respective pair of openings. The shutters comprise rigid massive bodies of resistant material having a density relative to water (i.e., greater than one). The shutters are pivoted between their ends on levers for movement in a restricted arc on a vertical plane. As the liquid level immerses each shutter, that shutter pivots on its lever to block the passage of light through its respective pair of openings. More specifically, as the volume of liquid in the column increases the number of shutters blocking the number of corresponding pairs of openings also increases, thus blocking the transmission of light through the corresponding pairs of openings. On the other hand, as the volume of liquid in the column decreases, the number of shutters blocking corresponding pairs of openings decreases, thereby allowing the transmission of light through the unblocked pair of openings. Like the Vasel device, the Stanley apparatus does not address the problem of variations in the fluid volume due to changes in fluid temperature. Similarly, since the shutters are arranged in the reservoir, the reservoir is complicated to construct and maintain.